1. Field of the Invention
This invention relates to digital data communication, and in particular to a technique for coherently demodulating asynchronous phase modulated carrier signals transmitted through a noisy medium.
2. Prior Art
"Base band" (high and low or "non-return-to-zero") digital data signals may be communicated through a medium such as an AC power line by providing a carrier wave in the range of 100 kHz to 400 kHz, modulating the carrier with the data signals, and adding the modulated carrier to the transmission medium. Carriers have been modulated in various ways. Amplitude and frequency modulation (AM and FM) are common because they are relatively simple, but they have the drawbacks of high error rates and wide bandwidths. These drawbacks are minimized by phase modulation (PM), also known as phase shift keying (PSK).
Referring to FIG. 1, a carrier signal A may be phase modulated by combining the carrier signal and a base band data signal B through an exclusive OR (XOR) gate. At each transition in the data signals, the gate phase-shifts the carrier signal 180.degree. to produce a PSK modulated signal C. A phase shift indicates a transition between data bit values, but does not indicate the bit values themselves. The bit values are indicated by the length of time between transitions. For example, a high level data bit can be indicated by a short (high or low) signal, and a low level data bit by a long (high or low) signal. A receiver measures the time between carrier phase shifts to reconstruct the original data. Phase modulation is a conventional technique, but up until the present invention, it has required such elaborate circuitry for modulation and demodulation that it has not been economical. There remains a need for a better technique to communicate digital data signals in the presence of heavy noise, particularly random, spurious, and impulse type noises.